1. Field of the Invention
This invention relates to a method and a device wherein, in controlling a position of a driven member by use of an absolute position detector, a backlash is corrected which serves as a principal cause of an error occurring between a detected position of a predetermined portion in a driving source, a transmission device or the like and an actual position of the driven member.
2. Description of the Prior Art
In general, in controlling a position of a driven member moved by a driving source and a transmission device, it is necessary to detect the position of the driven member, however, it is rarely the case that an actual position of the driven member is directly detected.
From this reason, there has heretofore been indirectly detected the actual position of the driven member by a detector provided on the driving source or the transmission device. However, a mechanical section such as the transmission device is interposed between a portion detected by the detector and the driven member, and, when the moving direction is changed, an error due to a so-called backlash occurs, so that the detected position in the transmission device and the like need not necessarily coincide with the actual position of the driven member.
For example, in FIG. 5, a ball screw 11 as being the transmission device is moved by a driving source, not shown, and its position is detected by a detector, while, a table 12 is moved by the ball screw 11. FIG. 5 shows the state after the transfer thereof to the right, and a backlash width B formed between the ball screw 11 and the table 12 exist to the left of the ball screw 11. Here, when the table 12 is moved to the right by a distance D, if the ball screw 11 is moved by a distance D to the right as shown in FIG. 6, the table 12 is moved similarly. However, when the moving direction is changed to move the table 12 to the left by the distance D, as shown in FIG. 7, even if the ball screw 11 is moved by the distance D to the left, the table 12 moves only by (D-B) due to the presence of the backlash width B. In order to correct the shortage of the backlash width B due to the change in the direction, an excessive movement commensurate to the value of the backlash width B is made, only when the moving direction is changed. The correction of the type described has heretofore been called the backlash correction.
Now, as a detector for detecting a position of the ball screw 11, there has heretofore been used an increment type position detector. The increment type position detector is adapted to ouput a relative movement value as the detected data, and, when an absolute position serving as a reference position is not given, it has been necessary to set a reference point prior to the action, i.e. at the time of a power up. For this, in the device using the conventional increment type position detector, immediately after the power up, an origin returning action is performed to set the reference point in a numerically controlled machine tool, thus enabling to detect the position. Further, at the time of the power up, it is unclear that the positional relationship between the ball screw 11 and the table 12 within the range of the backlash width B is. However, in the case immediately after making the above-described origin returning action, there is no backlash between the ball screw 11 and the table 12, so that the positional relationship between the ball screw 11 and the table 12 is unambiguously determined.
Now, it is wasteful in the terms of time to perform the origin returning action at the time of each power up, which has been a considerable obstacle for the improvements in the working efficiency. In this respect, the absolute position detector, which has been widely used, is advantageous in that the detector has higher accuracy, and the position can be detected immediately after the power up, so that the origin returning action can be dispensed with.
However, when the origin returning action is to be dispensed with at the time of the power up by the use of the absolute position detector which is connected to the ball screw 11, the position of the ball screw 11 may be detected immediately after the power up, however, when the table 12 is shifted in position from the ball screw 11 within the range of the backlash width B at the time of the power up, it is impossible to determining the position of the table 12 within the range of the backlash width B, thus resulting in the disadvantage that an error commensurate to the value of the backlash width B at the maximum occurs between the detected position of the ball screw 11 and the actual position of the table 12 as being the driven member.